HYDRA. lof) 



as the head, and has for a mouth the aperture of 

 the tube, which is furnished at its margin with a 

 single row of tentacula. It thus corresponds to the 

 general definition of a polypus, and exemplifies its 

 most simple form. 



The whole body may, on the one hand, be con- 

 siderably elongated, and on the other, so much 

 retracted, as to appear a mere globule ; and these 

 movements are the effect of a voluntary power in 

 the animal directed to specific ends. The number 

 of tentacula varies from six to twelve : they are 

 slender tubular filaments, capable of being extended 

 to a great length, and of being bent in all direc- 

 tions. In this way, they can quickly surround and 

 grasp any small object which they may happen to 

 touch ; and whenever irritated, they instantly re- 

 tract, so as hardly to be visible without the aid of 

 a magnifier. Each tentaculum may be moved 

 independently of the rest, at the pleasure of the 

 animal. The remainder of the body tapers gra- 

 dually from the head to the other extremity, be- 

 coming very slender, and having at its termination 

 a flat surface, which has been termed the foot : 

 for although every portion of the surface has the 

 power of adhering to the bodies to which it is 

 applied, it is principally by this extremity that the 

 animal chooses to attach itself to the sides or bottom 

 of the vessel in which it is kept. No trace of the 

 existence of cilia is to be met with on any part of 

 the surface of these animals. 



It is to Mr. Trembley of Geneva that we are 

 indebted for the discovery of this singular animal, 

 the examination of which has contributed to throw 

 great light on the natural history of polypiferoiis 



